Display renamed files in diff views

Show both the old and new filenames when viewing the diff for a renamed file, as suggested on [feedback.gitlab.com](http://feedback.gitlab.com/forums/176466-general/suggestions/4130669-display-moved-files-with-a-single-line-instead-of).

This depends on a change to gitlab_git; file rename detection is enabled there in [merge request 6](https://gitlab.com/gitlab-org/gitlab_git/merge_requests/6) on gitlab.com.  However, this change shouldn't break the existing diff view in the meantime.

Here are screen prints of some commits from the gitlab-shell repo.  The "before" view is the same with either of these combinations of gitlab and gitlab_git:

* `gitlabhq/master` gitlab and `gitlabhq/master` gitlab_git
* `mr-vinn/diff-renames` gitlab and `gitlabhq/master` gitlab_git

The "after" view requires `mr-vinn/diff-renames` gitlab and `mr-vinn/rugged-diff-options` gitlab_git

### Before

![diff_without_rename](https://cloud.githubusercontent.com/assets/2551678/4688449/0b60b052-5678-11e4-8fe0-07da00546e7b.png)

### After

![diff_rename_enabled](https://cloud.githubusercontent.com/assets/2551678/4688451/185a7126-5678-11e4-8d0a-f14acd78e0bb.png)